The present invention relates to automatic focusing photographic cameras, in general, and to control systems for focusing alternatively employed interchangeable adjustable focus lens assemblies on such cameras, in particular.
Photographic cameras having interchangeable adjustable focus lenses are well known in the prior art. Cameras having an adjustable focus lens and having a focus control system coupled to said lens for automatically focusing image forming light rays from a remote subject at the film plane of such a camera in response to a range finder derived signal representative of the actual distance between said camera and said remote object, are also known in prior art. A camera having such a focus control system and utilizing acoustical energy to determine the distance to a remote object is described in U.S. Pat. No. 3,522,764 to BIBER et al. In said BIBER et al patent separate transmitting and receiving transducers are employed in the ranging systems disclosed therein to determine subject distance. In U.S. Pat. No. 4,081,626 to MUGGLI et al and in U.S. Pat. No. 4,199,246 to MUGGLI, a single transducer that both transmits a burst of sonic energy toward a subject and subsequently detects an echo of said energy reflected from said subject is employed for object distance determining or ranging purposes.
The lens of a camera, as well as other optical instruments, has a characteristic viewing angle associated therewith that spans the area that is visible through said lens and is generally referred to as the lens optical field-of-view angle. Similarly, a highly directional, object presence detecting transducer for use in a focus control system in an autofocusing camera such as the combination transmitting and receiving transducer described in certain of the above-cited patents, also has a characteristic transmitting angle associated therewith that spans the primary region within which the presence of objects can be detected and that angle is referred to herein as the transducer main lobe or acceptance angle.
When selecting the size of a transducer acceptance angle for compatibility with an automatically focused adjustable focus lens, normal practice is to select an angle that is substantially smaller than the field of view angle of said adjustable focus lens, said acceptance angle usually being in the neighborhood of 10% of the lens field-of-view angle. If the transducer acceptance angle was greater than the field of view angle of the adjustable focus lens that it helps to focus, a spurious object detection signal may be generated from objects located outside of the adjustable focus lens optical field-of-view and closer to said control system transducer than objects located within said optical field of view, which might result in a misfocused lens. This problem is avoided by selecting an object detecting capacitance-type transducer with a relatively narrow acceptance angle in accordance with the criteria mentioned above.
One disadvantage associated with the requirement that a transducer acceptance angle be in the neighborhood of 10% of the field-of-view angle of the lens that it helps to focus is that in a photographic camera of the type employing interchangeable adjustable focus lens assemblies with different optical field-of-view angles an acoustical transducer is required whose acceptance angle can readily be varied to meet the 10% criterion mentioned above. In the capacitance-type transducer described in the above cited MUGGLE patent, the transducer angle can only be changed by either changing transducer operating frequency or by changing the diameter of the backplate that forms a portion of said transducer Changing the operating frequency will change the level or intensity of transmitted acoustical energy at any given distance from the transducer and would require more complex electrical circuitry to provide the object detecting means and the range of frequencies required for such an arrangement. To change the diameter of a backplate in a capacitance-type transducer previous practice would dictate substitution of one transducer with the appropriate acceptance angle for one that did not have the appropriate acceptance angle.
A primary object of the present invention is to provide a capacitance-type acoustical transducer having a variable acceptance angle.
Another object of the present invention is to provide a capacitance-type acoustical transducer whose effective backplate diameter can readily be changed by means external of said transducer.
A further object of the present invention is to provide an autofocus camera having a plurality of interchangeable, adjustable focus lenses with a focus control system having an object detecting acoustical transducer whose acceptance angle is automatically or manually changed to accommodate each such lens as or after it is attached to the camera body.
Other objects, features and advantages of the present invention will be readily apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.